In silver halide photography one or more radiation-sensitive emulsion layers are coated on a support and imagewise exposed to electromagnetic radiation to produce a latent image in the emulsion layer or layers. The latent image is converted to a viewable image upon subsequent processing.
Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element to X-rays. In 1913 the Eastman Kodak Company introduced its first silver halide photographic element specifically intended to be exposed by X-radiation--i.e., its first silver halide radiographic element.
The medical diagnostic value of radiographic imaging is accepted. Nevertheless, the desirability of limiting patient exposure to X-radiation has been appreciated from the inception of medical radiography. Silver halide radiographic elements are more responsive to longer wavelength electromagnetic radiation than to X-radiation. As herein employed the term "longer wavelength electromagnetic radiation" or "emitted radiation", except as otherwise qualified, indicates electromagnetic radiation in the 300 to 1500 nm spectral range, including both the near ultraviolet and blue regions of the spectrum to which silver halide possesses native sensitivity and the visible and near infrared portions of the spectrum to which silver halide is readily spectrally sensitized. Low X-ray absorption by silver halide radiographic elements as compared to absorption of longer wavelength electromagnetic radiation led quickly to the use of intensifying screens. The Patterson Screen Company in 1918 introduced matched intensifying screens for Kodak's first dual coated (Duplitized.RTM.) radiographic element. An intensifying screen contains on a support a fluorescent phosphor layer that absorbs the X-radiation more efficiently than silver halide and emits to the adjacent radiographic element longer wavelength electromagnetic radiation in an image pattern corresponding to that of the X-radiation received.
The need to increase the diagnostic capabilities of radiographic imaging while minimizing patient exposure to X-radiation has presented a diligently addressed challenge of long standing in the construction of both radiographic elements and intensifying screens. In constructing intensifying screens the ideal aim is to achieve the maximum longer wavelength electromagnetic radiation emission possible for a given level of X-radiation exposure (which is realized as maximum imaging speed) while obtaining the highest achievable level of image definition (i.e., sharpness or acuity). Since maximum speed and maximum sharpness in intensifying screen construction are not compatible, actual screens represent the best attainable compromise for their intended application.
The choice of a support for an intensifying screen illustrates the mutually exclusive choices that are confronted in screen optimization. It is generally recognized that supports having a high level of absorption of emitted longer wavelength electromagnetic radiation produce the sharpest radiographic images. Intensifying screens which produce the sharpest images are commonly constructed with black supports or supports loaded with carbon particles. Often transparent screen supports are employed with the intensifying screen being mounted in a cassette for exposure along with a black backing layer. In these screen constructions sharpness is improved at the expense of speed by failing to direct to the adjacent radiographic element a portion of the emitted longer wavelength electromagnetic radiation that might otherwise be available for latent image formation.
If a black or transparent intensifying screen support is replaced by a more reflective support, a substantial increase in speed can be realized. The most common conventional approach is to load or coat a screen support with a white pigment, such as titania or barium sulfate. Juliano U.S. Pat. No. 3,787,238, Degenhardt U.S. Pat. No. b 4,318,001, and Ochiai U.S. Pat. No. 4,501,971, are offered as illustrative only, since the majority of well drafted patents describing intensifying screen constructions mention at least in passing similar options for support construction.
Even the best reflective supports identified by the art for intensifying screen construction have degraded image sharpness in relation to imaging speed so as to restrict their use to applications less demanding of image definition. Further, many types of reflective supports that have been found suitable for other purposes have been tried and rejected for use in intensifying screens. For example, the loading of intensifying screen supports with optical brighteners, widely employed as "whiteners", has been found to be incompatible with achieving satisfactory image definition.
By a process of trial and error over a development period of approximately 70 years the intensifying screen art has developed a bias for the selection of reflective supports from a relatively limited class of constructions and against regarding as suitable for intensifying screen construction support elements that, though nominally reflective, were developed for other, less demanding purposes.
During the last quarter century, a period in which the potentially deleterious effects of even low levels of X-radiation exposure have been publically called into question and a period in which every obvious expedient and a virtual continuum of inventions have been pressed into service to increase the capabilities of diagnostic radiographic imaging while reducing patient X-ray exposure, there has existed in the art a class of reflective supports that have never been suggested for use in intensifying screens, hereinafter referred to as "stretch cavitation microvoided" supports.
In 1964, Johnson U.S. Pat. No. 3,154,461, disclosed a polymeric film loaded with microbeads of calcium carbonate of from 1 to 5 .mu.m in size. By biaxially stretching the support, stretch cavitation microvoids were introduced, rendering the support opaque.
Primary interest in stretch cavitation microvoided supports has centered on imparting to polymer film supports paper-like qualities, as illustrated by Takashi et al U.S. Pat. No. 4,318,950; Toyoda et al U.S. Pat. No. 4,340,639; Ashcraft et al U.S. Pat. Nos. 4,377,616 and 4,438,175; and H. H. Morris and P. I. Prescott, "White Opaque Plastic Film and Fiber for Papermaking Use," ACS Div. Org. Coatings Plastic Chemistry, Vol. 34, pp. 75-80, 1974.
More recently, stretch cavitation microvoided supports have been investigated as possible replacements for photographic print supports, as illustrated by Mathews et al U.S. Pat. Nos. 3,944,699 and 4,187,113 and Remmington et al U.K. Patent Specifications 1,593,591 and 1,593,592. Polypropylene microbeads are in one instance employed, but the preferred microbeads are white pigment barium sulfate microbeads.
Pollock et al U.S. Ser. No. 47,821, filed May 5, 1987, titled SHAPED ARTICLES FROM POLYESTERS AND CELLULOSE ESTER COMPOSITIONS, commonly assigned, discloses stretch cavitation microvoided shaped articles, such as films, sheets, bottles, tubes, fibers, and rods, wherein the polymer forming the continuous phase is a polyester and the microbeads are a cellulose ester.
From the 1960 filing of Johnson U.S. Pat. No. 3,154,461 until this invention there has been no suggestion that stretch cavitation microvoided supports would be suitable for the demanding requirements of radiographic intensifying screens.